The present invention is a method and apparatus for measuring the distance between two surfaces. The preferred embodiment illustrates the method and apparatus applied in the field of hard-disk magnetic recording for performing head/disk spacing or “flying height” measurements with substantially improved accuracy at very low flying heights compared to the current industry-standard methods.
Hard disk drives contain recording heads that read and write magnetic data to a rotating disk. The recording heads contain aerodynamic features that create an air bearing which controls the separation between the heads and disk. The thickness of the air bearing, or spacing between the head and disk is commonly referred to as “flying height.” The flying height greatly impacts the performance of a head. As flying height is reduced, data density on the disk can be increased significantly. However, the head must not contact the disk excessively or wear may occur, possibly leading to read/write failure. In order to verify head design and monitor production processes, the flying height of hard drive magnetic heads is frequently measured on a flying height tester.
Flying height testers commonly use a rotating transparent replica of the magnetic disk together with an actual magnetic recording head. In general, a measurement beam (1 or more) from a light source is projected through the disk onto the air bearing surface of the recording head and the reflected beam is analyzed to derive the flying height.
The current industry standard, DFHT IV (Dynamic Flying Height Tester-IV) distributed by the KLA-Tencor Corporation (Lacey U.S. Pat. No. 5,280,340) uses 1 multi-wavelength light source at normal incidence to the disk to produce interference at the head/disk interface. Three wavelengths are filtered from the reflected light and the intensities are analyzed to derive the flying height. This technique requires calibration of the light source intensity and the reflectance of the disk and air bearing surfaces. The calibration process involves mechanically separating the head from the disk. A perfect calibration requires the head to remain parallel to the disk and not translate during the separation process. Unfortunately, it is not possible to perform this calibration perfectly, without introducing any error. As the flying height decreases, the calibration error causes a greater error in flying height measurement. This condition has made many users aware that the accuracy of the DFHT IV flying height tester is not sufficient for many current and future head designs. While the DFHT IV flying height tester has been a great commercial success over the past 10 years, its commercial viability is becoming questionable for the ever-decreasing flying heights that are becoming common in the industry.
De Groot, U.S. Pat. No. 5,557,399, discloses a flying height tester that was distributed by the Zygo Corporation under the name Pegasus. The Pegasus flying height tester used polarization interferometry to measure flying height. The technique involves directing a laser at an oblique angle through the disk onto the head. The polarization state of the beam is known before it enters the disk. The beam reflected from the head is analyzed for polarization and phase changes and flying height is derived from that data. This technique has two significant difficulties. (1) The stress in the spinning glass disk causes birefringence that affects the data and is difficult to compensate for in calculations. (2) The oblique angle of incident light makes it difficult to locate the measurement spot with the required precision. Perhaps because of these reasons, the Pegasus flying height tester had little commercial success.
Lacey, et al. U.S. Pat. No. 5,638,178 disclose an FHT using an imaging polarimeter. This technique was also adversely affected by stress-induced birefringence in the disk. While this technique theoretically provided advantages over the DFHT IV flying height tester distributed by the KLA-Tencor corporation, its performance was sub-par and it was not accepted in the industry.
Sides, U.S. Pat. No. 5,715,060, (the 060 patent) discloses a method for measuring flying height by sensing scattered light from frustrated total internal reflection. A light beam is directed through the edge of the disk onto the head air bearing surface. The incident angle of the beam striking the head is selected to produce total internal reflection. As described in “Calibration of Fly Height Measured by Scattered Total Internal Reflection” by Strunk, Low and Sides (IEEE Transactions on Magnectic Vol 36, No 5, September, 2000), the total internal reflection produces an evanesent wave on the far side of the transparent disk, where the head flies. Any material very close to the disk may interact with the evanesent wave, frustrating the total internal reflection. In particular, a typical magnetic recording head will frustrate the total internal reflection and furthermore, scatter some of the light which would have otherwise been totally internally reflected. In the 060 patent, Sides teaches that the scattered light intensity can be measured by a photo detector. The intensity of the scattered light changes as the spacing between the disk and magnetic head changes, thus this effect can be used to measure flying height. The practical difficulty in applying this technique occurs when calibrating the scattered light. Several methods are suggested in the 060 patent but they all have significant disadvantages. One method requires contacting the head and disk which can be distructive. Another method is flying the head at a known height, which requires a separate flying height measurement the method of which is unknown, or using a calibration standard that potentially does not have sufficient resolution. Even if the calibration issue was addressed, it is not clear that this technique would provided a significant improvement compared to the commercially available DFHT IV flying height tester's performance at today's very low flying heights.
Guzik, U.S. Pat. No. 5,932,887, teaches a method for measuring flying height by sensing frustrated total internally reflected light. A light beam is directed through the edge of the disk onto the head air bearing surface. The incident angle of the beam striking the head is selected to produce total internal reflection. The head material frustrates the total internal reflection. The amount of reflected light is measured and flying height is calculated. This technique has several difficulties: it is not easy to capture the internally-reflected light. The requirements for the light entry into and exit from the disk place restrictions on the disk hub design and the positions where the head may fly on the disk. Furthermore, the signals produced by the system are difficult to translate into a very accurate flying height, especially at the very low flying heights of today's head designs.
Duran, U.S. Pat. No. 6,184,993, teaches the use of a Savart plate to split an interferometric image of the head/disk interface into polarized ordinary and extraordinary beams. These beams were then retarded with respect to each other by shifting optical components in the apparatus. After shifting, the beams are superimposed and their interference is detected. The interferometric intensity is detected at a variety of phase shifts, then the phase at a giving system configuration is calculated from the data. This phase is compared to the phase measured with no head present and the flying height is calculated from the resultant phase measurements. As disclosed, this method has limitations on the possible measurement locations on the head, as the superimposed images must meet in a specific way to make the measurement. It also may be difficult to control system drift as the phase would be significantly affected by very small optical path variations caused by thermal expansion and other factors. In any case, this system has yet to prove commercial viability as it has been over five years since the patent was filed.
The present invention overcomes most of the limitations of the prior art. It does not require a spacing-varying calibration as does the current industry-standard DFHT IV flying height tester distributed by the KLA-Tencor corporation. It also does not make use of polarized light, so it is immune to the difficulties caused by stress-induced birefringence in the spinning glass disk. Furthermore the technique can be performed at substantially normal incidence to the head/disk interface so the precision of spot location is not compromised as it can be on non-normal incidence systems. Another advantage of this technique is that the sensitivity increases as the spacing approaches zero, whereas the sensitivity decreases as the spacing approaches zero on the current industry-standard DFHT IV flying height tester.